1. Technical Field
The present invention relates to a constant voltage regulator, and more particularly, to a constant voltage regulator for power supply circuitry in electronic devices, such as personal computers and cellular phones, implementable in a low-current consumption integrated circuit (IC), which converts an input voltage input to an input terminal thereof into a regulated, output voltage output to an output terminal thereof.
2. Description of the Background Art
Voltage regulators are employed in power supply circuitry of various electronic devices, such as personal computers and cellular phones, which converts an input voltage input to an input terminal thereof into a regulated, output voltage for output to load circuitry, such as a microcontroller or other electronic components.
FIG. 1 is a circuit diagram schematically illustrating a configuration of a known constant voltage regulator 101.
As shown in FIG. 1, the voltage regulator 101 comprises a series regulator that converts an input voltage Vi supplied between an input terminal 111 and a ground terminal 112 to a regulated, constant output voltage Vo output to an output terminal 113.
The voltage regulator 101 includes a driver transistor M111, being a p-channel metal-oxide semiconductor (PMOS) device, having a source terminal thereof connected to the input terminal 111 and a drain terminal thereof connected to the output terminal 113; a pair of voltage divider resistors R111 and R112 connected in series between the output terminal 113 and the ground terminal 112 to form a feedback node therebetween; a reference voltage generator 116 connected to the ground terminal 112; and a differential amplifier EA111 having a non-inverting input thereof connected to the voltage divider node, an inverting input thereof connected to the reference voltage generator 116, and an output thereof connected to a gate terminal of the driver transistor M111.
During operation, the driver transistor M111 conducts an electric current therethrough according to a voltage applied across its gate and source terminals, so as to output a regulated output voltage Vo to the output terminal 113. The voltage divider resistors R111 and R112 generate a feedback voltage Vfb proportional to the output voltage Vo at the feedback node therebetween, whereas the reference voltage generator 116 generates a reference voltage Vref for comparison with the feedback voltage Vfb.
The differential amplifier EA111 compares the feedback voltage Vfb and the reference voltage Vref, so as to generate an error-amplified signal VEA at the output thereof by amplifying a difference between the differential input voltages Vfb and Vref. The amplifier output VEA thus generated is applied to the gate terminal of the driver transistor M111 to control operation of the same, thereby regulating the output voltage Vo to a desired, constant level.
With reference to FIG. 2, which is a detailed circuit diagram of the voltage regulator 101 of FIG. 1, the differential amplifier EA111 is shown including a differential pair of n-channel metal-oxide semiconductor (NMOS) transistors M112 and M113, the former having its gate terminal connected to the reference voltage generator 116, and the latter having its gate terminal connected to the feedback node between the voltage divider resistors R111 and R112; a current-mirror active load formed of a pair of PMOS transistors M114 and M115, the former connected in series with one differential transistor M112, and the latter connected in series with the other differential transistor M113, both having their gate terminals connected together to the drain terminal of the transistor M115; and a negatively-biased NMOS transistor M116 having one terminal grounded and another terminal connected to the differential pair to conduct a control current I111 therethrough.
To meet energy efficiency requirements of today's low-power consumption electronic devices, the voltage regulator 101 is required to operate with an extremely low current consumed through its differential amplification circuitry. To this end, the control current I111 of the differential amplifier EA111 is designed sufficiently small in amplitude, typically on the order of 500 nanoamperes to 5 microamperes, so as to reduce electronic current flowing through the multiple transistors.
FIGS. 3A and 3B are waveform diagrams showing the power supply input and output voltages Vi and Vo in volts (V), respectively, of the constant voltage regulator 101, each plotted against time in seconds (sec) during activation of the power supply circuitry.
As shown in FIGS. 3A and 3B, upon power-on, the input voltage Vi starts to rise at time t1, followed by the output voltage Vo rising toward a rated, constant level determined by the configuration of the reference voltage generator and the voltage divider resistors, which is typically 3.3 V with an allowance of ±10% for microcontroller applications. As the input voltage Vi continues to rise, the output voltage Vo reaches the rated output voltage at time t2, and then stops increasing to stabilize at the rated level at time t3.
During such initial stage upon power-on of the voltage regulator 101, the output voltage Vo upon reaching the rated level experiences a sharp, transient rise above the rated level, referred to in the art as “overshoot”. Such voltage overshoot occurs due to a response delay caused where the voltage regulator 101 takes time to control the gate-to-source voltage of the driver transistor M111 from an initial, high level to an operational, low level approximately equal to a threshold voltage of the transistor M111 upon detecting that the feedback voltage Vfb reaches the reference voltage Vref.
Although typically encountered where the power supply voltage suddenly increases upon power-on, such phenomenon also takes place in today's low-power consumption electronics even where the power supply voltage exhibits a relatively large time constant larger than which is determined by the driver transistor's ON resistance and load current, as well as capacitance connected to the output terminal of the voltage regulator. If not corrected, voltage overshoot above the maximum allowable limit of the output voltage can result in runaway or other failures of the load circuit supplied therewith.
To date, various techniques have been proposed to provide overshoot-protected voltage regulation for power supply with a rise time of several microseconds per voltage, as described below with reference to FIGS. 4 through 7.
For example, one known technique provides a voltage regulator 401 as shown in FIG. 4. This voltage regulator 4011 includes a differential amplifier 430 to compare a feedback voltage Vfb against a reference voltage Vref to generate an error-amplified output signal to a regulator output terminal Vo provided with a stabilizer capacitor 461.
According to this method, the voltage regulator 401 also includes a comparator 440 to compare the feedback voltage Vfb against the reference voltage Vref, which outputs a result of comparison for activating and deactivating a switch or discharge circuit 450 connected between the output and ground terminals. When activated, the discharge circuit 450 causes the capacitor 461 to discharge electricity, so as to prevent excessive voltage overshoot upon startup of the power circuitry.
One drawback of this method is that overshoot protection provided by the comparator 440 and the discharge circuit 450 does not effectively work, where the comparator 440 exhibits a certain amount of offset voltage that causes a delay in responding to voltage overshoot. Moreover, the voltage regulator 401 requires a substantial amount of current consumed by the comparator 440 to obtain prompt comparator response for effective overshoot protection, which, however, makes it difficult to implement the voltage regulator 401 in an integrated circuit (IC) that consumes low current during operation.
Another known technique provides an overshoot protection circuit including a capacitor and resistors connected to an output of a voltage regulator, which monitors the output voltage to withdraw electric current from the output terminal upon detecting a transient change in the output voltage.
Such method has a drawback in that it requires a large value or size of capacitor and resistors forming the overshoot protection circuit to properly protect against voltage overshoot, where the power supply voltage as well as the output voltage rise upon power-on with a time constant larger than that which is determined by the driver transistor's ON-resistance and load current, and the capacitance connected to the output terminal. Due to such size requirement for the capacitor and resistors, which makes it difficult to implement the voltage regulator on a single IC, this method remains impractical or otherwise unduly expensive to practice.
Still another known technique provides a voltage regulator 501 as shown in FIG. 5. This voltage regulator 501 includes a driver transistor M511 connected between input and output terminals 511 and 513; a pair of resistors forming a voltage divider 506 connected to the output terminal 513 to output a feedback voltage; a reference voltage generator 516 to output a reference voltage Vref; and a differential amplifier EA511 having its differential inputs connected to the voltage divider 506 and the reference voltage generator 516, respectively, to output a control signal to a gate terminal of the driver transistor M511.
According to this method, the voltage regulator 501 also includes a soft start circuit 519 formed of a resistor and capacitor connected between the output of the reference voltage generator 516 and the input of the differential amplifier EA511, which provides the output of the reference voltage generator 516 with a time constant determined by the resistance and capacitance connected therewith, so as to protect the output voltage from excessive overshoot where the input voltage suddenly increases upon power-on.
As is the case with the overshoot protection circuit depicted above, such method has a drawback in that it requires a large value or size of capacitor and resistor forming the soft start circuit to properly protect against voltage overshoot, where the power supply voltage rises upon power-on with a time constant larger than that which is determined by the driver transistor's ON-resistance and load current, and the capacitance connected to the output terminal. Due to such size requirement for the capacitor and resistor, which makes it difficult to implement the voltage regulator on a single IC, this method remains impractical or otherwise unduly expensive to practice.
Still another known technique provides a voltage regulator 601 as shown in FIG. 6. This voltage regulator 601 includes a driver transistor Q611 connected between input and output terminals 611 and 613 to conduct a drain current Io therethrough; a pair of resistors forming a voltage divider 606 connected to the output terminal 613 to output a feedback voltage; a reference voltage generator 616 to output a reference voltage; and control circuitry formed of a differential amplifier EA611 having its differential inputs connected to the voltage divider 606 and the reference voltage generator 616, respectively, to output a control signal to a gate terminal of the driver transistor Q611.
According to this method, the voltage regulator 601 also includes a current limiter 619 connected to the input terminal 611 which limits the drain current of the driver transistor Q611 to protect the output voltage from excessive overshoot where the input voltage suddenly increases upon power-on.
Such method has a drawback in that it cannot effectively protect against voltage overshoot in case the drain current flowing through the driver transistor remains extremely low, for example, where the power supply voltage rises upon power-on with a time constant larger than that which is determined by the driver transistor's-ON resistance and load current, and the capacitance connected to the output terminal.
Yet still another known technique provides a voltage regulator 701 as shown in FIG. 7. This voltage regulator 701 includes a driver transistor M711 connected between input and output terminals 711 and 713; a pair of resistors forming a voltage divider 706 connected to the output terminal 713 to output a feedback voltage; a reference voltage generator 716 to output a reference voltage; a differential amplifier EA711 having its differential inputs connected to the voltage divider 706 and the reference voltage generator 716, respectively, to output a control signal to a gate terminal of the driver transistor M711; and a current limiter 742 to limit a current passing through the driver transistor M711.
According to this method, the voltage regulator 701 also includes a control transistor M753 connected between the source and gate terminals of the driver transistor M711, and an RC low-pass or high-pass filter consisting of a resistor 751 and a capacitor 752 connected in series to the input terminal 711, with a node therebetween connected to the gate terminal of the control transistor M753, which together form a time constant circuit that charges a transistor parasitic capacitance Cp as the filter detects a sudden change in the input voltage, so as to protect the output voltage from excessive overshoot where the input voltage suddenly increases upon power-on.
A similar method is proposed to provide overshoot protection with a low-pass or high-pass filter connected to a bias circuit that determines a control current supplied to the differential amplifier, wherein the bias circuit temporarily increases the control current as the filter detects a sudden change in the input voltage, so as to protect the output voltage from excessive overshoot where the input voltage suddenly increases upon power-on.
Either of such methods using a filter-based overshoot detector has a drawback in that it requires a large value or size of capacitor and resistor forming the RC filter to properly protect against voltage overshoot, where the power supply voltage rises upon power-on with a time constant larger than that which is determined by the driver transistor's ON-resistance and load current, and the capacitance connected to the output terminal. Due to such size requirement for the capacitor and resistor, which makes it difficult to implement the voltage regulator on a single IC, this method remains impractical or otherwise unduly expensive to practice.